1. Field of the Invention
The invention generally relates to pipeline ADC (Analog to Digital Converter) units, and in particular to WLAN (Wireless Local Area Network) communication devices such as transmitters, receivers and transceivers, and corresponding integrated circuit chips and methods, where pipeline ADC units are used for converting analog transmission and/or reception signals to digital data.
2. Description of the Related Art
A wireless local area network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, wireless LANs transmit and receive data over the air, minimizing the need for wired connections. Thus, wireless LANs combine data connectivity with user mobility.
Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: FHSS (Frequency Hopping Spread Spectrum) and DSSS (Direct Sequence Spread Spectrum) systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to the 802.11b standard that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible as far as it relates to direct sequence spread spectrum technology, but it adopts a new modulation technique called CCK (Complementary Code Keying) which allows the speed increase.
Further extensions to the IEEE 802.11 standard exist. For instance, the IEEE 802.11a and 802.11g specifications use the OFDM (Orthogonal Frequency Division Multiplexing) technique which is a wireless transmission technique that splits signals into sub signals that are then transmitted at different frequencies simultaneously. The 802.11g version of OFDM uses a combination of BPSK (Binary. Phase. Shift. Keying), QPSK (Quadrature. Phase. Shift. Keying), and QAM (Quadrature. Amplitude. Modulation), depending on the chosen data rate.
Thus, a variety of different modulation types and methods exist within 802.11 compliant WLAN systems. Not all of the possible transmission modes have to be supported by each individual WLAN device, but multi-mode WLAN devices exist that support at least part of the possible modes.
As WLAN systems are digital data communication systems, communication devices in such systems use ADC units to convert analog signals to digital data. One of several different kinds of analog to digital converters are pipeline ADC units. Pipeline ADC units provide an optimum balance of size, speed, resolution, power dissipation, and analog design efforts. Also known as subranging quantizers, pipeline ADC units consist of numerous consecutive stages. An example of a conventional pipeline ADC unit is depicted in FIG. 1.
As apparent from this figure, the pipeline ADC unit comprises M-1 stages 100–120 and a digital correction unit 130 to convert an incoming analog signal to an M-bit digital output signal. The stages 100–120 often include ADC/MDAC (Multiplying Digital to Analog. Converter) circuits, and there may also be a sample/hold amplifier at the input side of the pipeline.
An example of an individual conventional stage architecture is shown in FIG. 2. As can be seen, each stage has a sample/hold amplifier 200, a coarse ADC 210 to approximate the input signal, and an MDAC circuit 220 together with a summing circuit 230 to subtract the quantized signal from the input. This difference is then amplified by an amplifier 240 to provide the analog residue that is forwarded to the next stage.
Thus, referring back to FIG. 1, each stage receives an analog signal from the preceding stage, outputs an analog signal to the succeeding stage, and delivers the digital data downwards to the digital correction unit 130. The digital correction unit 130 corrects the offsets in the quantizers, i.e. in the ADC units within the MDAC circuit.
One of the possible pipeline ADC architectures is a pipeline 1.5b/stage architecture with nine stages, i.e. M=10. In this architecture, each stage generates two bits with the sub-ADC 210, and amplifies the resulting residue by a gain of 2. The sample and hold function may then be realized by buffering switch-capacitor gain blocks allowing concurrent processing. The resulting 18 bits are delayed accordingly and combined with digital correction to yield a 10-bit digital output signal.
While such pipeline ADC units have a high data resolution which may be useful for instance in 802.11g compliant WLAN systems, the units suffer from the rather high power consumption. Taking a multi-mode WLAN communications device, this high power consumption then also applies in modes such as 802.11b compliant WLAN modes, where the 10-bit accuracy is not required. Thus, multi-mode WLAN devices would need both a 10-bit pipeline and a, e.g., 6-bit pipeline, to adjust power consumption and digital resolution to the individual operational mode. However, this does lead to significant circuit development and manufacturing costs so that this approach is frequently of no practical use.
Also when considering single-mode WLAN communication devices, the circuit developers need to create and maintain circuit designs for various pipeline ADC structures of different lengths. This makes the handling with such circuits more cumbersome and is quite inefficient in practice.